With the recent development of the information society, it has been sought to increase an integration density and operation speed of a magnetic recording/reproducing device representing a magnetic disk device, and to reduce the size of the magnetic recording/reproducing device on the market. As perpendicular magnetic recording scheme can meet these needs. The perpendicular magnetic recording scheme is suitable for an increase in a surface recording density in principle. In the perpendicular magnetic recording scheme, the higher a linear recording density of a pattern recorded on a magnetic disk, the smaller the intensity of a diamagnetic field and the more the intensity of magnetization is stabilized. In addition, in the perpendicular magnetic recording scheme, a component (leaking in a direction of a track width) of a magnetic field generated from a recording head, is small.
As shown in FIG. 16, a perpendicular magnetic recording head is structured by laminating a read unit 14 and a write unit 15. The read unit 14 has a lower shield layer 8, a read element 9 and an upper shield layer 10. The read element 9 is located between the lower and upper shield layers 8 and 10. A portion of the read element 9 is exposed to an air bearing surface. A giant magnetoresistance effect head, a tunnel giant magnetoresistance effect head capable of providing a large reproduction output, a current perpendicular to plane (CPP) giant magnetoresistance effect head capable of causing a current to flow perpendicularly to a film surface, or the like, may be used as the read element 9. The write unit 15 has a magnetic gap formed on the side of the air bearing surface. The write unit 15 has a main magnetic pole 13 and an auxiliary magnetic pole 11. The main magnetic pole 13 and the auxiliary magnetic pole 11 are magnetically coupled with each other on the opposite side to the air bearing surface. The write unit 15 also has a coil 12 provided between the main magnetic pole 13 and the auxiliary magnetic pole 11. A magnetic shield 16 is provided around the main magnetic pole 13. A soft magnetic underlayer 18 is provided under a recording layer 17 included in the magnetic disk 2 since it is necessary that a component (measured in a direction perpendicular to the surface of the magnetic disk 2) of a magnetic field generated from the main magnetic pole 13 be used to record data in a perpendicular magnetic recording scheme. The surface of the soft magnetic underlayer 18 faces the surface of the main magnetic pole 13. Therefore, a high-intensity component (measured in the direction perpendicular to the surface of the magnetic disk 2) of a magnetic field can be generated. A magnetic flux present in the soft magnetic underlayer 18 is returned by the auxiliary magnetic pole 11 and circles.
In order to realize a high recording density, it is necessary to realize a high linear recording density (BPI) and a high track density (TPI). To increase the BPI, it is necessary to improve the gradient of a magnetic field generated from a write head and improve resolution of a read head. In order to increase the TPI, it is necessary to reduce a track width of the write head and reduce magnetic side writing. Japanese Patent Publication No. 2007-35082 (“Patent Document 1”) discloses a technique for a perpendicular magnetic recording head. The perpendicular magnetic recording head disclosed in Patent Document 1 has a trailing shield and a side shield around a main magnetic pole in order to reduce magnetic side writing and improve a magnetic field gradient. A first nonmagnetic film is provided between the main magnetic pole and the trailing shield, while a second nonmagnetic film is provided between the main magnetic pole and the side shield. The first nonmagnetic film is made of a material different from a material of the second nonmagnetic film, and formed by a method different from a method used to form the second nonmagnetic film. This improves accuracy of the thickness of the first nonmagnetic film, although the thickness of the first nonmagnetic film may affect the intensity of a magnetic field. Therefore, the technique disclosed in Patent Document 1 realizes the perpendicular magnetic recording head having a small track width, and allows the perpendicular magnetic recording head to be produced in large quantities.
As described above, in order to achieve a high recording density by means of the perpendicular magnetic recording head, it is effective to reduce a track width of the main magnetic pole, form a nonmagnetic gap that is located between the main magnetic pole and the trailing shield and has a length (film thickness) with high accuracy. It is, however, found out that the following problems to be solved further exist through study conducted by the present inventors and other persons. A write head constituting a part of the perpendicular magnetic recording head is formed on a wafer by a thin film formation process such as sputtering, ion milling, and photolithography. In order to form a main magnetic pole of the write head, a magnetic film is formed, and a mask member is formed on the magnetic film. Then, a mask pattern is formed by reactive ion etching (RIE). The magnetic film is then subjected to ion milling using the mask pattern. In this way, the main magnetic pole of the write head is formed. Since etching at an outer peripheral portion of the wafer progresses more easily than etching at a central portion of the wafer during the RIE based on characteristics of the RIE, the width of a central portion of the mask pattern present on the wafer is large, and the width of an outer peripheral portion of the mask pattern present on the wafer is small. If the ion milling is performed using this mask pattern, the width of a central portion of the main magnetic pole is large, and the width of an outer peripheral portion of the main magnetic pole is small. Therefore, even when the same wafer is used, perpendicular magnetic recording heads are manufactured, which are provided with main magnetic poles that are located at the central portion of the wafer and at the outer peripheral portion of the wafer and have respective widths different from each other. The variation in the widths of the main magnetic poles may cause a variation in recording characteristics. It is therefore necessary to reduce the variation in the widths of the main magnetic poles.
Japanese Patent Publication No. 6-275730 (“Patent Document 2”) describes the following. That is, in a process for forming a multi-layer wiring using an organic film as an interlayer insulating film, when Ar ion milling is performed in order to remove a metal oxide from the surface of an underlying metal wiring layer, a non-uniform distribution tends to occur on the surface of a wafer forming a multi-layer wiring substrate, and the Ar ion milling tends to be performed on a central portion of the wafer at high speed and on an outer peripheral portion of the wafer at low speed. Even when the metal oxide is removed from the surface of the underlying metal wiring layer at the central portion of the wafer, the metal oxide may remain on the surface of the underlying metal wiring layer at the outer peripheral portion of the wafer. Thus, the metal oxide may be non-uniformly removed from the surface of the underlying metal wiring layer present on the wafer. To solve the problem, an oxide layer formed on the surface of the underlying metal wiring layer is removed by reactive ion etching using an Ar gas and ion milling using an Ar gas, i.e., by combining reactive ion etching using an Ar gas having a tendency of the opposite in-plane distribution of the etching rate with ion milling using an Ar gas having the tendency. In addition, Japanese Patent Publication No. 2003-78185 (“Patent Document 3”) discloses a method for uniformly controlling an etching depth in a substrate in the following etching process. In the process of etching an upper ferromagnetic layer included in a ferromagnetic tunnel junction structure having a body formed by laminating a lower ferromagnetic layer, a tunnel barrier layer and the upper ferromagnetic layer, and having a magnetic bias layer formed above the laminated body via a gap layer, a portion ranging from the magnetic bias layer to a part of the gap layer is processed by using ion milling. Then, the gap layer remaining after the ion milling is removed by reactive ion etching to ensure that the upper ferromagnetic layer is exposed. After that, the upper ferromagnetic layer is processed by ion milling. Each of Patent Documents 2 and 3 discloses that the reactive ion etching and the ion milling are combined to uniformly control the etching depth in the wafer or the substrate. However, Patent Documents 2 and 3 do not describe that when ion milling is performed in a process of forming a main magnetic pole of a perpendicular magnetic recording head after reactive ion etching, the width of the main magnetic pole at a central portion of a wafer is large and the width of the main magnetic pole at an outer peripheral portion of the wafer is small. Furthermore, Patent Documents 2 and 3 do not describe a method for controlling the variation in the width of the main magnetic pole.